Emulsion explosive compositions are well-known in the art. They are fluid when formed (and can be designed to remain fluid at temperatures of use) and are used in both packaged and bulk forms. They commonly are mixed with ammonium nitrate prills or ANFO to form a "heavy ANFO" product, having higher energy and, depending on the ratios of components, better water resistance than ANFO. Such emulsions normally are reduced in density by the addition of gas or air voids in the form of hollow microspheres or gas bubbles, which increase the sensitivity of the emulsion to detonation. A uniform, stable dispersion of the microspheres or gas bubbles is important to the detonation properties of the composition. The gas bubbles normally are produced by the reaction of chemical gassing agents.
Chemically gassed emulsion explosive compositions are well-known in the art. See, for example, U.S. Pat. No. 4,960,475 and the patents referenced therein. Chemical gassing agents normally are soluble in the inorganic oxidizer salt or discontinuous phase of the emulsion and react chemically in the oxidizer salt phase under proper pH conditions to produce a fine dispersion of gas bubbles throughout the emulsion. The timing of the addition of the gassing agent is important. The gassing agent or portion thereof that decomposes or reacts chemically in the oxidizer salt solution generally cannot be added to the oxidizer salt solution prior to formation of the emulsion or gassing would occur prematurely. Similarly, if an emulsion is to be subjected to further handling procedures, such as pumping into a borehole or mixing with ammonium nitrate prills or ANFO, then the chemical gassing reaction should not occur fully until after such handling occurs in order to minimize coalescence and/or escape of the gas bubbles. Further, after final placement of the explosive into a borehole, package or other receptacle, gassing should progress to completion in a desired time frame for the specific application or subsequent activities such as cooling, packaging or borehole stemming could interfere with the desired density reduction. Thus the gassing timing and rate must be optimized for a given application. U.S. Pat. No. 4,960,475 discloses the use of a gassing surfactant to accelerate the rate of gassing even in emulsions containing stabilizing polymeric emulsifiers. As used herein, the term "chemical gassing agent" shall include all components that are added to the emulsion explosive composition to produce gas bubbles.
In addition to a faster gassing rate, there are other characteristics that are desirable in a chemically gassed emulsion explosive composition. These are: (a) increased stability of the gassed composition, (b) reduced gas bubble size and (c) increased dispersion of the gas. These characteristics are important to the performance parameters of the composition, such as stability, sensitivity and detonability, and thus can be determinative of its commercial viability.
In the present invention, the addition of an appreciable amount of calcium ion (Ca.sup.2 +) or strontium ion (Sr.sup.2 +) to a nitrite salt chemical gassing agent has been found to improve the stability of a gassed emulsion explosive composition (as used herein "stability" means the persistence of an emulsified state, i.e., no phase separation or crystallization of the internal phase); to increase its gassing rate; and to generate smaller and more finely dispersed gas bubbles, which are less susceptible to coalescence, provide good sensitization and result in a higher detonation velocity for the emulsion explosive composition. As will be shown in the examples that follow, the advantages obtained from adding calcium or strontium ion often are dramatic.